Depleted uranium is an extremely dense metal that has been used for years as the primary constituent of kinetic energy penetrators. Depleted uranium itself has a ductility of approximately 8-22% and a relatively low tensile strength of 67-102 ksi; rolled and heat-treated depleted uranium has 12-49% elongation and a tensile strength of 83-109 ksi. The requirements for a successful penetrator, however, call for a material having significantly higher strength to assist penetration in addition to a density greater than 18 gm/cc to provide a maximum amount of kinetic energy, and high ductility so the penetrator will not bend or shatter on impact. Accordingly, uranium alloys have been used for penetrators.
There has been some effort made to modify the mechanical properties of uranium to improve its strength while maintaining sufficient ductility. Heat treatment, alloying and thermomechanical processing techniques have been used to improve the strength of depleted uranium. Metallurgical approaches to strengthening that have been shown to be operative in uranium include grain refinement, substructure refinement, strain hardening, precipitation strengthening and dispersion strengthening. The alloying elements that have been studied in uranium metallurgy include molybdenum, niobium, titanium and zirconium.
Perhaps the most commonly used alloy for penetrators is U-0.75 weight % Ti. It has been found that uranium-titanium alloys having about 0.6% to 0.8% titanium with appropriate heat treatment have a two-phase room temperature microstructure of alpha' uranium plus U.sub.2 Ti. The alloy in this condition has a yield strength of approximately 123 ksi (thousands of pounds per square inch), a tensile strength of approximately 200 ksi, and an elongation of 24%: for penetrator design, approximately 10% elongation is required. After peak aging treatment, the maximum yield strength is about 200 ksi, the tensile strength is about 215 ksi, but the elongation only 2%. Accordingly, the U-0.75% Ti alloy with sufficient ductility for penetrator use has a yield strength of well under 200 ksi.
In heat treating the U-0.75% Ti alloy, proper control of the quench rate is required in order to provide the proper mode of transformation that occurs upon cooling from the solutionizing temperature to room temperature. To achieve the desired 100% martensitic structure, in which the gamma to alpha transformation is suppressed and the gamma phase transforms directly to the desired alpha' acicular martensitic structure, the U-0.75% Ti alloy must be quenched at approximately 100.degree. centigrade per second from the approximately 800.degree. C. temperature of the gamma phase to room temperature. To achieve this quench rate, a combination of a water quench process and alloy section sizes of less than approximately 3 centimeters is required. Accordingly, the U-0.75% Ti cannot effectively be heat treated in section sizes greater than 3 centimeters and still achieve the required strength and ductility.
In general, as alloy content is increased, the martensite start transformation temperature of the alloy decreases, resulting in an increased quench rate sensitivity. This effect is very pronounced for molybdenum additions, and less pronounced for titanium additions. Accordingly, the overall effect of alloy content on quench rate sensitivity is a balance between the undesired suppression of the martensite start temperature and the retardation of diffusional transformations.
The U-0.75% Ti alloy is typically aged to increase strength and hardness at the expense of ductility. Strengthening is typically accomplished by aging in the temperature range 350.degree. C. to 450.degree. C., which results in precipitation strengthening without a large amount of cellular decomposition of the acicular martensite to the equilibrium alpha and U.sub.2 Ti phases. To achieve the best combination of strength and ductility in the U-0.75% Ti alloy, an underaging treatment of four to six hours at 380.degree. C. is most commonly used, producing an alloy with a yield strength on the order of 130 ksi and a ductility of over 10%.
Another uranium alloy, U-2 weight % Mo, exhibits highest ductility when processed in the overaged condition. For example, yield strengths of up to 130 ksi with ductility of over 10% can be achieved. However, for yield strengths greater than 130 ksi, ductility is extremely low as the alloy must be processed in the underaged or peak aged conditions. For example, at peak aged condition the yield strength is about 210 ksi, but the elongation is only about 1%.
A number of polynary uranium alloys have also been previously studied. Such alloys can be solutionized, quenched and age hardened in a manner similar to that for the U-0.75% Ti and U-2% Mo. However, these polynary alloys typically have a total alloy content of much greater than 2%, resulting in banded alpha" martensitic as-quenched structures that can be aged to high strength, but have very high quench rate sensitivity, low ductility, and increasingly lower density as the alloy content is increased. These alloys also have densities less than 18 g/cc, making them unsuitable for KE penetrator use. Accordingly, the known polynary uranium alloys do not have the combination of density, strength, quench rate sensitivity and ductility properties required for use as penetrators.